1. Field of the Invention
The invention relates to transistor switching circuits capable of safe operation at collector voltages in excess of the secondary breakdown voltage level of transistors contained in the transistor switching circuits.
2. Description of the Prior Art
Various high voltage breakdown phenomena are known to occur in transistors. High voltage breakdown can cause distruction of transistors operating in high voltage switching circuits, thereby causing circuit malfunctions in electronic systems containing the high voltage switching circuits. A number of high voltage breakdown parameters of transistors, including the open base collector-to-emitter breakdown voltage (BV.sub.CEO), the base shorted collector-to-emitter breakdown voltage (BV.sub.CES) and the punch through voltage (V.sub.PT) are well known and have been widely described in the literature.
It is also well known that transistors experience a phenomena referred to as secondary breakdown. Secondary breakdown occurs at collector voltages which are lower than the above mentioned high voltage breakdown parameters, but the actual collector voltage at which secondary breakdown occurs depends upon the operating conditions of the transistor as well as the physical parameters of the transistor. Power transistors are frequently characterized in their written product specifications by "safe operating area" (SOA) characteristics. The SOA characteristic for a particular transistor can be experimentally derived and constitutes the locus of points (for a given set of operating conditions) within which it can be guaranteed that the transistor will not go into secondary breakdown during a turn-off operation switching the transistor from an "on" condition to an "off" condition. Thus, the SOA curve (or curves) for a transistor shown the highest permissible collector voltages of that transistor during its operation in a switching circuit. Secondary breakdown has been widely discussed in the literature, although the mechanisms involved are not yet completely understood. See for example: "Avalanche Injection and Second Breakdown in Transistors", by Phillip L. Hower & V. G. K. Reddi, IEEE Transactions on Electron Devices, Vol. ED-17, No. 4, April, 1970; "Second Breakdown in Power Transistors Due to Avalanche Injection", by Brent A. Beatty, Surinder Krishna, Michael Adlar, IEEE Transactions on Electron Devices, Vol. ED-23, No. 8, August, 1976; "Power Transistor Crystal Damage in Inductive Load switching": A Reliability Concern", by F. P. Gaur, G. Lowe, W. Thorpe, Proceedings of 1977 International Reliability Physics Symposium, Las Vegas, Nev., Apr. 12, 1977; "Second Breakdown of Transistors during Inductive Turn-off", by S. Krishna, P. L. Hower, Proceedings of IEEE, Letters, March, 1973, p. 393-395; "Load Line Shaping Considerations for the High Speed Switching Transistor", by W. R. Skanadore, Proceedings of Powercon 4, May, 1977; U.S. Pat. Nos.: 3,418,495 to Bose; 3,736,495 to Calkin et al.; 3,628,047 to Cronin et al.
High voltage power switching transistors having BV.sub.CBO characteristics up to approximately 450 volts are readily commercially available. However, for power transistors of a particular power dissipation and current handling capability, cost increases sharply as the BV.sub.CEO voltage level increases. Further, power transistors having high BV.sub.CEO levels have undesirable corresponding characteristics, such as high saturation voltages.
As mentioned above, the secondary breakdown voltage level ordinarily is substantially less than BV.sub.CEO for a particular transistor of the type under discussion. However, if it can be assured that the transistor will not go into secondary breakdown during its operation, then BV.sub.CBO, rather than the secondary breakdown voltage or BV.sub.CEO represents the highest voltage which may occur on the collector of a transistor during its operation as a "switch" (which herein means that the transistor is turned on and off during its operation).
In this case, a circuit designer can utilize BV.sub.CEO specifications rather than SOA curves to select a suitable power transistor for such applications. Unfortunately, circuit designers have found it very difficult to reliably avoid secondary breakdown in high power transistors operated as switches. Consequently, circuit designers ordinarily select switching power transistors having a minimum secondary breakdown voltage level which exceeds any expected collector voltage during operation of the transistor in the particular application, since if secondary breadkown does occur in the transistor, it will usually be destroyed. It should be noted that a power transistor having a minimum secondary breakdown voltage level is ordinarily substantially less expensive than one having that voltage level as its BV.sub.CEO or BV.sub.CBO level.
Power transistors ordinarily have an emitter region having a relatively large rectangular region and a number of elongated fingers. A metal layer having a number of corresponding elongated fingers contacts the fingers of the emitter region and also includes a relatively large rectangular emitter bonding pad which lies over and is in electrical contact with the rectangular portion of the emitter. It is well known that a high ratio of peripheral emitter length to emitter area is desirable for power transistors. However, a finger-like structure would be unfeasible beneath the emitter bonding pad, so a rectangular portion of the emitter is used beneath the emitter bonding pad.
Accordingly, it is an object of the invention to provide transistor circuitry which avoids occurrence of secondary breakdown for collector voltages greater than the secondary breakdown voltage level of transistors contained in the switching circuits.
Another object of the invention is to provide a method of operating transistors in switching circuits to avoid secondary breakdown while the collector voltages of the transistors exceed the secondary breakdown voltage levels of the transistors.
It is another object of the invention to provide an improved transistor having a structure which makes the transistor less susceptible to secondary breakdown than previous transistor structures.
It is yet another object of the invention to provide a power transistor structure which is less prone to secondary breakdown and is readily manufacturable utilizing conventional transistor manufacturing techniques.